Fall Fishkill

The bass are belly-up at Peña Blanca Lake.

Peña Blanca Lake south of Tucson, once the queen of Southern Arizona's quartet of waterways, has had her tiara bent. Again.

The lake, oldest of the four fishing ponds in Southeastern Arizona, has had her crown tarnished before by everything from toxic chemicals leaching out of old mining claims to a massive buildup of silt and on-going drought conditions that have now lowered the water level as much as 5-6 vertical feet. When the levels went down, the fish started coming up--dead--beginning about mid-August.

"These floaters were generally largemouth bass in the 3- and 4-pound category," said Don Mitchell, fisheries biologist with the Tucson Game and Fish office. "We were seeing up to 40 or 50 dead or dying fish each day, and there were probably many more that never made it to the surface because of the lake's thick weed cover."

The fish kill was one of those natural phenomena that avid anglers dread, when a lake's oxygen content becomes depleted to the point that its fish population pays the price.

Mitchell said that when wildlife managers started noting increased sightings of dead fish about two months ago, they took water-quality samples and discovered oxygen levels had diminished to the point where the fishery's survival was in question.

"The lake was way down, and then a series of storms quickly filled it up, stirring up anoxic (oxygen deficient) sediment layers on the bottom," says Mitchell.

As a result, nutrient-rich sediment started decomposing and burning off more oxygen, while lingering clouds shut down the photosynthesis process that creates oxygen during daylight hours.

"I'm not willing to make any predictions on the ultimate casualty count, because estimates of the severity right now would only be a guess," Mitchell said.

Although he won't speculate on the full extent of the fall fish kill, he says "the worst may be over," unless sudden cooler nights hasten the annual winter thermocline shift, in which cold and warm layers of lake-water temperature invert.

The lake was created in 1957 behind an earthen dam built in a narrow canyon by Arizona Game and Fish, and it has been a watery magnet for birds, animals and human beings ever since. It once covered more than 50 surface acres, but heavy silting from major storm run-off has shrunk usable lake size by 10 percent. Because the lake is so well-nourished, shoreline vegetation proliferates, forming a thick surface mat on the water and contributing even further to the photosynthesis problems.

Its waters have also been tainted by traces of mercury, washed down from 100-year-old gold and silver mines in the adjacent Pajarito Mountains. That situation caused the Arizona Department of Environmental Quality several years ago to issue a public advisory cautioning against consumption of warm-water game fish--the first such recommendation ever issued in Arizona in response to potential mercury health concerns.

To remedy that problem, old mine-tailing ponds were dug up four years ago, and more than 1,000 tons of contaminated soil were carted off to prevent additional inflow.

A search of historical records indicates this year's conditions have resulted in the first recorded fish kill at Peña Blanca in at least three decades, coming at a time when the lake was just starting to earn angling kudos.

"Fishing was starting to improve, and it was not beyond the realm of possibility to catch half a dozen bass that would weigh in the 4- and 5-pound range," said Mitchell,

Although not officially weighed and recorded by G&F, Mitchell also cites what he calls "credible" reports 8-pound bass caught late this summer. Before the fish kill, Mitchell estimated that 10-pound-plus largemouth might soon be caught in the lake.

"I don't think the current situation at Peña Blanca will be anywhere near as bad as the back-to-back devastating fish kills at nearby Arivaca Lake in 1999 and 2000," he said. A similar mix of decaying vegetation, low water levels, increased water temperature, and heavy cloud cover turned the neighboring lake a pea-soup-green that stunk of rotten eggs and killed off thousands of big bass.

Looking for largemouth became a pastime of the past for angling regulars as only a few smaller fish survived.

"Big bass took a huge hit in these kills," Mitchell said, noting an estimated 20,000 fish carcasses littered the shoreline. It took until the fall of 2001 for water conditions to improve enough to re-stock what anglers used to call "the Honey Hole." The re-stocking and a catch-and-release requirement have helped nurse that stricken waterway back to health.

"Anglers are having a lot of success at Arivaca with consistent reports of bass in the 4- and 5-pound range and an occasional lunker as heavy as 8 pounds," said Mitchell.

Fish and Game biologists are continuing to monitor improving Peña Blanca water quality conditions, but a return to normal could take weeks or months. Rainbow trout stockings, scheduled to begin in November, have been canceled for the month--and through the end of the year if necessary.

"My crystal ball is broken," said Mitchell, "but resumption of trout stocking here won't proceed until conditions improve. Those hatchery-raised trout, some 4,500 in November that were originally intended for Peña Blanca, will now go to Patagonia and Parker Canyon Lakes."

(Note to anglers: Get to both lakes in a hurry, as largemouth bass in Patagonia like to fatten up for the winter on stocked trout, and illegally introduced northern pike in Parker Canyon gather for feeding when the trout supply truck pulls up.)

"We're hoping for a speedy and successful recovery at Peña Blanca, similar to what has taken place at Arivaca," said Mitchell.

Visitors can reach Peña Blanca the easy way by taking Interstate 19 south to the Ruby Road Exit and heading west on a rolling two-lane blacktop for about 10 miles to the parking lot. Those who have a greater sense of adventure, with no concerns about time and speed, as well as heavy-duty shocks, can reach the lake via the unpaved portion of Ruby Road. This is a graded-gravel experience accessible by taking the I-10 Arivaca exit and traveling through working cattle ranches, cottonwood trees and abandoned buildings of ghost towns such as Ruby and Camp McKee until the lake signs appear.